[daip] image versus u,v plane stacking

[Eric Greisen]

Andrew Biggs wrote:
> Hi Eric. I have a quick question regarding stacking in radio images. I've
> done this with my GMRT data and got a detection so it all seems to work.
> However, I saw a talk by some people in Sweden who claim that "u,v
> stacking" is superior. By this I think they mean the equivalent of running
> UVFIX on each source to place them at the phase centre and then combining
> them into a single dataset (presumably with some hefty time averaging to
> keep the data size reasonable). They claim that the SNR of their detection
> doubles, but I don't really understand this as I would have thought that
> the two techniques would give the same answer. I was wondering if you had
> any comment on this - before I go to the bother of writing a script to do
> this in AIPS.

There is already a script that should help - RUN STUFFR and HELP STUFFR

This converts the data times into hour angles, does DBCON, and the 
averages the data in time in a baseline-length dependent mode.  It has 
been used to put together multiple days on the same field primarily.

The image value at the origin is simply the sum of the visibilities 
(assuming no Clean).  Summing images or summing visibilities should be 
exactly equal - at the origin.  I guess I could see how uv stacking 
might help away from the origin.  There are issues - the UBAVG averaging 
depends on the u,v,w of a baseline/source staying similar for a while 
but if you have multiple sources combined that will not be the case.  I 
guess if the sources were separated into separate subarrays, then the 
baseline sort would separate them.  Perhaps you need to "stuff" each 
source separately and then combines them.

Eric